This invention relates to the maufacture of television cathode ray tubes and is concerned specifically with electrically resistive materials for use in such tubes.
It is conventional in television cathode ray picture tubes to provide an electrically conductive coating comprised of colloidal graphite deposited on the inner surface of the funnel. The conductive coating commonly covers the entire inner surface of the funnel in an area extending to the junction of the funnel and the neck of the picture tube, as is well known in the art. A conductive coating is also located on the outside surface of the funnel. The glass of the cathode ray tube funnel serves as the dielectric medium of a capacitor formed by the difference in electrical potential established between the outer conductive coating and the inner conductive coating. The outer conductive coating is commonly at ground, or zero potential, and the inner conductive coating is at a relatively high potential, typically about 30 kilovolts. This high potential is applied to the forward electrode, or "anode electrode," of the electron gun located in the neck of the tube by a plurality of "snubber" springs extending from the gun to make contact with the inner conductive coating. By exerting a uniform outward pressure, the snubber springs also serve to center the forward section of the gun in the neck.
The high potential on the inner conductive coating and the propinquity of the electron gun parts has resulted in the problem of arcing primarily between the anode electrode and one or more of those electrodes of the electron gun adjacent to the anode electrode. The peak energy density of an arc typically may exceed several hundred million watts/CM.sup.2, causing a literal explosion to take place within the tube. The effect is a loud report and oftentimes damage to the gun and ancillary television circuits, especially those with transistors. The severity of some arcs and the release of energy implicit therein can actually crack the neck of the tube.
To alleviate this tendency toward destructive arcing, there has been established the practice of introducing a highly resistive coating intermediate to the inner conductive coating and the area adjacent the anode electrode of the gun. The purpose is to control the surface potential of the glass neck and suppress current build-up during arcing. The result has been to reduce the incidence of arcing, or if arcing does occur, reduce the magnitude of the arcing currents to a tolerable level.
Several factors combine to impose stringent requirements on the composition and physical characteristics of the resistive coating. The material must provide very high resistance, of the order of several megohms. The composition must be homogeneous to prevent the existence of paths of low resistance, or voids of infinite resistance where there is a dearth of the resistive component. The material must be tolerant to heat experienced during manufacture and operation. Also, the material must be readily adherent to the glass of the type used in cathode ray tubes. Resistance to scratching by the snubber springs when the gun is inserted in the neck of the tube is another requisite as particles generated by such scratching can degrade tube quality.
Further, the material must be compatible with standard cathode ray tube processing procedures such as air bake during fritting and vacuum bake during the exhaust cycle, and subsequent high-voltage conditioning or "spot-knocking." Also, the material must be of low vapor pressure so it will not out-gas during the operating life of the tube. The ability to resist burn-out and consequent loss of electrical continuity at the points of contact with the snubber springs is yet another requirement.
We are aware of a method of compounding a resistive glass frit which comprises the method of dry mixing and blending an antimony oxide with tin oxide in the ratio of about 1 to 100, firing the mixture for two hours at a temperature of no more than 1300.degree. C. in a quartz crucible having a loosely fitted, porous cover, and mixing the resulting powder with about an equal amount of glass frit. We found the resistive values of the frit compounded by this method to be unpredictable and non-reproducible.
A resistance coating consisting of an amorphous layer of an homogenous compound of a glass composition material, with at least one particulate material selected from a group basically consisting of cadmium oxide, indium, and copper oxide, is disclosed by German Pat. Nos. 27 49 210, and 27 49 212.
General background in the art, together with a description of a resistive coating, is presented in a paper by J. W. Schwartz and M. Fogelson entitled: "Recent Developments in Arc Suppression For Picture Tubes." (IEEE Transactions on Consumer Electronics, Volume CE-25, Feb. 1979).
A resistive formulation that can be used as a resistive coating is described and fully claimed in U.S. Pat. No. 4,153,857 to Delsing and Fogelson, assigned to the assignee of this invention. The resistive element is so widely and deeply cavitated and contorted at or below its nominal surface that the real surface of the element is shadowed and very greatly extended in area relative to the nominal surface of the element. The effect of this is that when the getter is flashed, the coating of conductive getter material deposited on the element is effectively dispersed and fragmented into isolated conductive islands. The result is to render tolerably insignificant the tendency of arc currents to travel over the surface of the element and thereby by-pass the body of the element. The surface topography is substantially that associated with the crystallization of camphor or the like.
In U.S. Pat. No. 4,101,803 to Retsky and Schwartz, assigned to the assignee of this invention, there is disclosed an anti-static coating deposited on the inner surface of the neck of a cathode ray tube around the beam egress from the electron gun and having a dynamic impedance value which is greater than that of associated arc-suppression resistors. The anti-static coating serves to drain off stray charges and to transmit the high voltage on the funnel inner conductive coating to the gun.